Voltage rating is only part of the story when it comes to capacitors. Ignition Service is rough duty. It isn't that they are a high-tech product, after all they've been around for 100 years now, but apparently modern manufacturers can't build them to a low price point any longer. Not in the small, compact package used when mounted internally inside a distributor anyway. The other criterion other than voltage rating is known as Dv or rise time. Kind of related to current, I expect.

So what you have to look at when actually choosing a capacitor for points & condenser applications is whether it can meet the ignition specs. There are very compact yet high voltage capacitors available made by metal vapor deposition, these are tempting to use because they will so easily fit inside distributors, but they simply won't hold up. Suitable capacitors tend to be a little on the chunky side, physically speaking. There's nothing that says it must be mounted inside the distributor, btw.

Thank you for your detailed responses! I understood that increased resistance would demand more voltage on the secondary side, but the nuances of the current/voltage/magnetic fields/oscillation on the primary side are pretty nebulous to me so I wasn’t sure whether the primary side would also see increased current or voltage in some way. I have found that some of my plug wires are over 32k ohms, and even the coil to cap wire is over 12k. Those resistances, along with the “E” type rotor on the distributor, must be working the ignition system harder than intended.

Before I install the pertronix I’m going to see if there is any evident change in spark quality from just changing the plug wires, just out of curiousity.

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

Those sparkplug wire resistances sound high to me. I just went out in the garage and measured a couple new spiral wound sparkplug wires.  They averaged about 3760 ohms per foot. If I can remember correctly, even the old carbon wires were not as high of a resistance as you reported, maybe others will chime in. When you get the new sparkplug wires it will be interesting to see what they measure.  Rotor and sparkplug gap is what the voltage must overcome. I don't know off hand what the size hole is in a Ford rotor but if you can find something that size to slip the rotor onto, a dial indicator can be used to compare the tip to that of a known good rotor.  I recently found the rotor in my distributor to be eroded by .010"  but that's another story for a new thread. 

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I should of added that the actual resistance with an ohm meter with its 3 or 4 volts across the probes is not a true indication of wire condition but the resistance I stated above is typical of what I'm used to seeing.  The resistance of the core is designed to reduce RMI and RFI interference.  The actual energy passes over the core (skin effect) rather than through the spiral core. Someone else with more knowledge than I on this subject could explain this much better.  

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https://www.magnecor.com/magnecor1/main.htm       Hopefully I did this correctly. If the link opens, on the left side of the screen click on "THE TRUTH A BOUT IGNITION WIRE CONDUCTORS".   It's a long article by Magnacore but very good..........................

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Sortof off topic.

I've started checking plug wire resistance when new and writing it down.  This  helps to confirm that the new wires have no problems but also provides a baseline for future trouble shooting.  Resistance per foot can vary wildly from one wire tech to another so knowing this from the start can be a big help.


Lawrenceville, GA

Charlie, can you fix my posted link so it will open?  I think this article will explain some of what you mentioned above along with what is important, what is not and why.  Thanks.............

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Fixed it.  There was an extra space at the end.


Lawrenceville, GA

Interesting reading from Magnecor. It is refreshing to read what appears to be a transparent, no-nonsense representation of the product and its appropriate application.

I measured the resistances of my new wire set and it looks like they are about 4500 ohms per foot. This is definitely an improvement over what I had on there, which was probably deteriorated from age, oil, antifreeze, temperature, etc.

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

Looks like things are headed in the right direction. When you have the new Pertronix unit and coil installed, I think you will put your ignition issues to bed for good.   Thanks Charlie for fixing the link. I'll probably be an analog person for ever...........

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